The following patent publications are believed to represent the current state of the art:
U.S. Published Patent Application No. 2008/0278353;
U.S. Pat. Nos. 7,270,275; 6,646,565; 6,917,299 and 4,486,637;
European Patent Nos.: 1421549 and 1676182; and
Great Britain Patent Application No. GB8608277.